Metal/air batteries have been studied for many years. The zinc/air battery is the most widely known and used. It is most commonly used in the form of small button cells as a power source for electronic hearing aids. Since the depolarizer, that is, the true cathode material is normally incoming air, the cell has high capacity and energy density for its size. The cell's anode is usually formed of a slurry of zinc particles wetted with an aqueous solution of alkaline electrolyte. The cathode material typically contains a small amount of a mixture of manganese dioxide particles, graphite and carbon black which mixture catalyzes the electrochemical reaction of incoming air and zinc. The anode reaction is Zn+2(OH)−1=ZnO+H2O+2e. The cathode reaction is ½O2+H2O+2e=2(OH)−1. The overall reaction for this cell is: Zn+½O2=ZnO. Although the aqueous alkaline electrolyte, typically an aqueous solution of KOH, has high ionic conductivity and is an excellent electrolyte for the system, there are some disadvantages in employing the aqueous electrolyte. Although there is no net water consumed or generated in the electrochemical reaction, a principal disadvantage is that once the cell is activated by allowing atmospheric air to enter the cell, the cell has a very short activated life. This is caused by the interaction between the electrolyte and atmospheric air, since the atmospheric humidity conditions in which the cell is operated is also a factor. In a low-humidity environment the water in the electrolyte tends to vaporize into the environment, causing the “drying out” phenomenon. In a high a high-humidity condition, the water vapor in the atmosphere tends to enter the electrolyte, causing a “flooding” phenomenon. Both situations interfere with cell performance and shorten the cell's activated life. In addition as atmospheric carbon dioxide enters the cell interior, it will gradually react with the aqueous KOH electrolyte to form potassium carbonate, K2CO3. The formation of carbonates interferes with achieving the desired cell performance. Such potential problems are not limited to the zinc/air cell but would be of concern in any metal/air cell employing an aqueous alkaline electrolyte.
Magnesium primary (non-rechargeable) batteries have been developed which employ an anode of magnesium alloy material and a cathode comprising manganese dioxide and acetylene black and an aqueous electrolyte. The cell may be referenced as a primary Mg/MnO2 cell. The aqueous electrolyte is an alkaline electrolyte which may contain magnesium perchlorate and magnesium hydroxide with barium and lithium chromate as corrosion inhibitors. The electrolyte is an aqueous electrolyte and the amount of water therein is important as the water participates in the anode reaction and is consumed during cell discharge. See, e.g. David Linden, Handbook of Batteries, Second Edition (1995), McGraw Hill, p. 9.2-9.3. During cell discharge the magnesium metal in the anode reacts with hydroxide (OH)− ions from the alkaline electrolyte resulting in production of magnesium hydroxide Mg(OH)2 which precipitates. The anode reaction is Mg+2(OH)−=Mg(OH)2+2e. The cathode reaction is 2MnO2+H2O+2e=Mn2O3+2(OH)−. The overall reaction for the cell can be written as Mg+2MnO2+H2O=Mn2O3+Mg(OH)2. The Mg(OH)2 is not soluble in the alkaline electrolyte and tends to form an insoluble passivation film on the surface of the magnesium anode as the cell discharges. The film passivates the cell's electrochemical activity. That is, the Mg(OH)2 film on the magnesium anode surface continues to retard the kinetics of the electrochemical discharge reaction as the cell is used, since the passivation layer continues to form on the magnesium anode surface during cell use and even during storage. The resulting effect is that the cell does not perform to its potential resulting in significant loss in capacity and power. Because of this problem the cell has not become popular.
Magnesium/air (Mg/air) primary cells have also been attempted. The anode comprises magnesium metal or magnesium alloyed with a minor amount of another metal. The Mg/air cell typically contains an aqueous alkaline electrolyte as above described with respect to the Mg/MnO2 cell. The anode is oxidized by incoming air which passes into the cell through an internal air diffusion filter. The incoming air passes through the cell's separator and contacts the magnesium anode allowing the electrochemical discharge reaction to occur. The cell's anode reaction is: Mg=Mg2++2e. The cathode reaction is ½O2+H2O+2e=2(OH)−1. The overall reaction is: Mg+½O2+H2O=Mg(OH)2 The cathode may include carbon material such as acetylene black which acts as a catalyst to improve the rate of electrochemical reaction between the magnesium anode and incoming air. In this respect the magnesium/air cell functions in manner similar to the zinc/air cell, which has a principal use as a hearing aid battery. However, unlike the zinc/air cell the magnesium/air cell has the same problem as above described with respect to the Mg/MnO2 cell, namely formation of a Mg(OH)2 passivation layer on the surface of the magnesium anode as the cell is discharged.
The formation of the Mg(OH)2 passivation layer on the magnesium anode significantly interferes with attainment of the desired cell performance. As a result the Mg/air cell has gained only very limited practical application. One such limited application of the Mg/air cell has been to develop and apply the cell as reserve power source for undersea applications, where the slight acidic nature of seawater and dissolved oxygen in the seawater function as electrolyte and cathode active material respectively. Such a battery has a long shelf life (sealed) because it remains dry in its unactivated state and does not become activated until immersed in sea water. That is, the battery does not initially contain water and becomes immediately activated on deployment when it is immersed into the sea water. The slight acidic nature of the sea water tends to neutralize the Mg(OH)2 as it forms in the cell during cell discharge.
In view of the problems and limited application of magnesium batteries having aqueous based electrolytes, there has been some effort to replace the aqueous based electrolyte with organic electrolyte for these batteries. However, this has proved to be a very difficult problem. Magnesium anode tends to react with non-aqueous electrolytes as well to form a film significantly passivating the electrochemical activity of the anode. Consequently, no practical prior art electrochemical cell has been achieved with magnesium anode based on non-aqueous electrolytes. Additionally, the non-aqueous electrolytes for application to magnesium cells have to date been insufficiently ionic conductive compared to aqueous electrolytes. That is, prior art non-aqueous electrolytes do not exhibit high enough ionic conductivity for the magnesium ions and do not provide sufficiently high ionic transport properties allowing the necessary transport of the magnesium ions therethrough. This results in loss of rate capability and tends to reduce the power output potential of magnesium cells when non-aqueous electrolytes are used to replace known aqueous electrolytes. Such limitations highlight the need to search for more conductive non-aqueous electrolyte systems or find improvements to existing systems in order to make the electrolyte more suitable for application to magnesium batteries.
U.S. Pat. No. 6,265,109 B1 discloses a metal/air battery wherein the anode comprises a magnesium metal preferably in the form of magnesium alloyed with a minor amount of another metal, namely, indium (In), gallium (Ga), tin (Sn), lead (Pb), cadmium (Cd), manganese (Mn), cobalt (Co), zinc (Zn), and thallium (Tl). The reference is directed principally at a magnesium/air cell. The possible inclusion of iron sulfide into the cathode is mentioned, but this is in connection with a specific organic electrolyte, namely, trimethylsulfoxide (TMSO) containing Mg(ClO4)2 dissolved therein at 1 mol per 1 mol of the solvent. (col 7, lines 7-14). Organic non-aqueous electrolytes are disclosed for use in the magnesium/air cell. The disclosed organic electrolytes may contain Mg(ClO4)2 salt dissolved in a solvent comprising an acid amide such as N-methylformamide or N,N-dimethylformamide and the electrolyte preferably comprises at least one other solvent selected from the group consisting of dimethylacetoamide, acetonitrile, ethylene carbonate, propylene carbonate, and γ-butyrolactum. Good results are reported. However, the utilization of such electrolytes, per se, does not prevent deleterious passivation layers from forming on the magnesium anode surface. Such passivation material in addition to Mg(OH)2 may also include MgO or Mg(CO3). The Mg(CO3) may develop in the magnesium/air cell as a result of reaction between magnesium ions and atmospheric carbon dioxide which enters into the cell along with the incoming air. The reference does not suggest additives for the electrolyte which would specifically retard the rate of formation of the passivation material on the magnesium anode surface in order to improve the cell's performance.
U.S. Pat. No. 6,991,876 B2 discloses a metal/active oxygen battery containing a non-aqueous electrolyte. The anode is preferably lithium, sodium, magnesium, or aluminum. The oxidizing medium is not atmospheric air but rather and oxygen containing species within the cell, thereby functioning as the cell's cathode. The active oxygen containing species may be organic in which case it is indicated to be covalently coupled to an organic peroxide-containing material. The active oxygen species may be ionically coupled to a metal, such as lithium, preferably forming lithium peroxide, Li2O2, which may be dispersed in a carbon-containing carrier. The disclosed non-aqueous electrolyte includes preferred electrolytes lithium hexafluorophosphate salt LiPF6 dissolved in solvent mixture comprising ethylene carbonate and demethyl carbonate, or a mixture of ethylene carbonate, dimethyl carbonate, and triethylphosphate. The reference, as indicated by the examples, is principally directed to lithium cells wherein the cathode contains active oxygen. Specific cells fabricated and tested are lithium/lithium peroxide (Li/Li2O2 ) cells as shown in the examples. The possibility of a magnesium/ lithium peroxide battery is also discussed briefly at col. 9.
There has also been some investigation on the possibility of employing magnesium insertion electrodes for rechargeable batteries which utilize non-aqueous electrolytes. The intent is to develop a battery which would allow the reversible formation of magnesium ions to form during cell discharge and revert back to magnesium during cell charging much in the same way as a lithium ion rechargeable battery operates. In such cell a suitable host electrode material into which the magnesium may be inserted is needed as well as suitable non-aqueous electrolyte which has the proper combination of properties of sufficient stability and conductivity. Possibilities for such a rechargeable system utilizing a magnesium negative electrode is explored in the reference: Petr Novak, Roman Imhof, and Otto Haas, Magnesium Insertion Electrodes for Rechargeable Nonaqueous Batteries—A Competitive Alternative to Lithium?, Electrochimica Acta, Vol. 45 (1999), 351-367. Although some possibilities have been investigated there is no commercial rechargeable cell of this type yet on the market.
Accordingly, it is desired to find a suitable non-aqueous electrolyte system for electrochemical cells employing anodes comprising magnesium or magnesium alloy as anode active material.
It is desired that the electrolyte system be suitable for cells having an anode comprising magnesium or magnesium alloy and either an air cathode or a cathode comprising iron disulfide.
In particular it is desired that the non-aqueous electrolyte system be suitable for cells having an anode comprising magnesium or magnesium alloy and cathode comprising iron disulfide.
It is desired to find an electrolyte system comprising a magnesium salt dissolved in a non-aqueous solvent mixture wherein the electrolyte system is suitable for the above indicated cells having an anode comprising magnesium or magnesium alloy, wherein one or more specific additives are added to the solvent mixture in order to retard formation of a passivation coating on the anode surface.